Currently, nitric acid is produced industrially via the catalytic oxidation of ammonia, over a platinum or platinum alloy-based gauze catalyst. This process, known as the Ostwald process, has essentially remained unchanged, since its inception in the first decades of the twentieth century. Ostwalds's patent was dated 1902 and when combined with Haber's development of synthesising ammonia, in 1908, the basis for the commercial production of nitric acid, which we use today, was in place.
The combustion of ammonia is carried out over a platinum-based metal or alloy catalyst in the form of a gauze or mesh or net. A number of gauzes are installed together, and they constitute the gauze pack. The upper-most gauzes have compositions optimised for the combustion of ammonia, and are referred to as the combustion gauzes. Gauzes with other compositions may be located below the combustion gauzes, and these may have other roles, as described below. The whole stack of gauzes is referred to as the gauze pack. The gauzes are produced either by weaving or knitting.
The operating temperatures of the plants are typically 830 to 930° C. and the range of pressures is from 100 kPa to 1500 kPa. Typically, the combustion gauzes are installed in the plant for between six months and two years, depending on the plant operating conditions. Plants operating at high pressures typically have shorter campaigns than low-pressure plants.
The duration of the campaign is governed by a loss in the selectivity of the catalyst, towards the desired nitric oxide product, through the increased formation of unwanted nitrogen and nitrous oxide by-products. The loss of selectivity is related to a number of phenomena. During combustion, platinum is lost through the formation of PtO2 vapour. Some of the platinum may be recovered by the installation of palladium metal-based gauzes, directly below the platinum-based combustion gauzes. The PtO2 vapour alloys with the palladium, therefore, platinum is retained in the catalytically active zone. However, due to the depletion of platinum in the upper combustion zone of the gauze pack, not all of the ammonia is immediately combusted. If the ammonia is combusted in the palladium gauze region, the selectivity towards nitric oxide is reduced, and secondly, if ammonia and nitric oxide coexist in the vapour phase for a period of time, nitric oxide is reduced by ammonia, through a homogeneous reaction. This leads to both nitric oxide and ammonia losses. A final mechanism for loss of selectivity is related to the fact that the platinum is lost from the combustion gauzes at a higher rate than the other alloying elements (typically rhodium). This leads to rhodium enrichment of the gauze surface which leads to selectivity loss.
Over the last sixty years, many attempts have been made to replace the expensive platinum-based combustion catalyst with lower cost catalysts, based for example on metal oxides. To date, the only commercially available oxide-based catalyst for ammonia combustion was developed by Incitec Ltd (Australia). This is based on a cobalt oxide phase. However, in terms of its selectivity of combustion of ammonia to the desired nitric oxide product, its performance is inferior to that of platinum-based systems. The cobalt oxide based systems have shown selectivity levels of circa 90%, in commercial units, compared to the 94 to 98% achieved with platinum-based catalysts.
The use of mixed oxides with the perovskite structure, such as rhombohedral lanthanum cobaltate, as catalysts for ammonia oxidation, has received much attention. However, when considering the conditions that the catalyst is subjected to in industrial ammonia oxidation, it can clearly be seen that they are not suitable for stability reasons. Ammonia oxidation on an industrial scale, takes place at temperatures from 830 to 930° C. and at pressures from 100 kPa to 1500 kPa. The concentration of ammonia is in the range of 8.5 to 12 mol %, depending on plant conditions, with the remainder of the gas consisting of air. Thus the gas feed for oxidation has a composition of approximately 10 mol % NH3, 18.7 mol % O2 and the balance being nitrogen. When the ammonia is oxidised to NOx (NO+NO2), with an efficiency of 95%, the gas composition is approximated by 9.5% NOx, 6% O2 and 15% water vapour (The balance of gas composition is nitrogen and some 800 to 2000 ppm of N2O). Thus the ammonia oxidation catalyst is subjected to high temperatures and a gas environment that contains oxygen and water vapour. These are the ideal conditions for the evaporation of metal ions, in the form of hydroxides and oxyhydroxides. Thus material will be lost from the catalytic reaction zone as vapour phase species, which will in-turn be deposited downstream in a cooler zone of the reactor system.
If considering evaporation from mixed oxides (those that contain more than one metal component), it most often has an incongruent evaporation process. This is the situation where one component in the oxide has a higher evaporation rate than another or than the others. If considering the lanthanum cobaltate perovskite system, when heated in an atmosphere containing oxygen and water vapour, cobalt species, such as CoOOH, have much higher vapour pressures than the dominant lanthanum species La(OH)3. The effect of this is that cobalt evaporates to a larger extent than lanthanum—thus incongruent evaporation. The result of preferential cobalt evaporation is that in time, the non-stoichiometry limit of the lanthanum cobalt perovskite X will be exceeded (LaCo1−XO3where X and 0<X≈<0.03). When the limit is exceeded, La2O3 will be precipitated. When operating, La2O3 does not have a negative effect on the catalyst performance. However, when the plant is shut-down or when it trips, the oxide catalyst is exposed to the ambient air. On cooling in air, the free-La2O3will hydrate; forming La(OH)3. One mole of La2O3will form two moles of La(OH)3, which involves a 50% expansion of the volume of the free-lanthanum species. This results in a mechanical disintegration of the catalyst.
Different perovskite type oxidation catalysts are known for use in different oxidation reactions. Examples of such catalysts and reactions are mentioned below.
WO 2006/010904 relates to an oxidation process wherein a perovskite oxidation catalyst of the formula ABO3 in which A is one or more metal cations selected from bismuth and the lanthanide metals and B represents one or more metal cations selected from the transition metals. Use of Y as described in our application is not mentioned. Particularly mentioned perovskite oxidation catalysts are GdCoO3 and GdCeCoO3. Ce is known to be rather hydroscopic. In ammonia oxidation applications more cobalt than “A-site” cations will be lost through evaporation. Therefore, at some stage A-site oxides will precipitate. When the reactor is periodically shut down, the catalyst is exposed to water vapour from the ambient environment. This will lead to hydration of susceptible metal oxides and to a physical/mechanical breakdown of the catalyst.
This patent application do also relate to oxidation processes and both oxidation of hydrocarbon and ammonia is mentioned. The problem they want to solve, however, is to reduce sulphur poisoning of perovskite oxidation catalysts, as sulphur poisoning reduces both activity and selectivity. The problem is solved when A and B cations in perovskite oxidation catalysts are selected so that at the operating temperature of the process a stable metal sulphate does not form.
In Baiker et al; “Influence of the A-site cation in ACoO3 (A=La, Pr, Nd and Gd) perovskite-type oxides on catalytic activity for methane combustion”, Journal of Catalysis (1994), 146(1), p. 268-76 the effect of rare earth ions (La, Pr, Nd and Gd) in ACoO3 perovskite-type catalysts on thermal behaviour and on catalytic activity for methane oxidation is discussed.
Zhao Fuhou, Lu Caiyun, Li Wan; “Rare earth element-containing perovskite-type catalysts for catalytic oxidation of pyridine”, Environmental Chemistry (1987), 6(4), 16-20. The catalytic effect of rare earth containing perovskite compounds in the oxidation of pyridine has been studied. DyCoO3, LaCoO3, DyMnO3 and GdCoO3 were found to have good efficiency in the conversion of pyridine.
Viswanathan et al., “Kinetics and mechanism of carbon monoxide oxidation on rare earth ortho-cobaltites”. Indian Journal of Technology (1984), 22(9), p. 348-52. A tentative mechanism for the catalytic oxidation of CO on LnCoO3 (Ln=La—Ho) was proposed.
Examples of other catalysts and reactions are mentioned below:
U.S. Pat. No. 3,888,792 describes a cobalt spinel oxide, Co3O4, combined with a scandium, yttrium or rare earth oxide support phase. Thus, this is composite material containing two or more oxides and where the cobalt in the active catalyst remains in the spinel structure and is not a catalyst with perovskite structure as described in the present invention. The catalyst can be used in industrial oxidation processes as for example ammonia oxidation, but none of the examples give any results comparable to the use of gauzes. There is no hint to how to obtain more efficiency in conversion rate or low levels of N2O. The problem they want to solve according to this patent is to obtain a catalyst with better mechanical strength, but the mechanical strength problem is different from in our application where swelling of the catalyst should be avoided.
Zhang et al, Journal of the Chinese Ceramic Society, Vol 40, February 2012, pages 289 to 293, describes a single phase mixed oxide containing yttrium (or yttrium and gadolinium), barium and cobalt Y1−XGdXBaCoO7+d. The structure is hexagonal with lattice parameters of circa 6.29 and 10.25 Å. The metal ratio between (Y+Gd): Ba: Co is 1:1:4, with oxygen content from 7 to 7.2. In this structure the yttrium and gadolinium have a 3+ valence state; the barium has a two plus valence state. For charge balance the average oxidation state of the cobalt is between 2.25 and 2.35. The structure also possesses four distinct metal sites within the lattice. One site occupied by yttrium and gadolinium, one site occupied by barium and two sites occupied by cobalt. This structure is very distinct from our Y1−XGdXCoO3 phase. The Y1−XGdXCoO3 may be considered to be an ABO3 phase, where the Gd and or Y occupy the A site and the cobalt occupies the B site. The average oxidation state of cobalt in the Y1−XGdXCoO3 is 3. Thus, both compositions, crystal structure and use is quite another than according to our invention. There is neither any hint to the use of these structures as catalytically active components.
US patent application 2012/0088936 describes a catalyst with a general formula Ln2MYCu1−X−YPdxO4+−d. This phase is classified as a Ruddlesden-Popper phase, which have a general formula An+1BnO3n+1, where n is an integer (i.e. it is an A2BO4 type structure). In the case of the US patent, n=1, A=(La, Pr, Nd, Sm or Eu) plus (Y, Ce, Yb, Ca, Sr or Ba), and M=Cr, Mn, Fe, Co, Ni and Al. The structure also contains copper and palladium. This phase is quite distinct from our Y1−XGdXCoO3, which has an ABO3 ortho-cobaltate structure. Thus, this patent describes a catalyst with another structure and there is no hint to that it could have been used as oxidation catalyst or especially for ammonia oxidation, either.